The press stem of both direct and indirect extrusion presses for light metals, heavy metals and steel is subjected to severe mechanical loading. The cross-section of the stem must be designed bearing in mind this loading and the strength of the material of which the stem is to be made.
When the stem is fixed rigidly at its foot, in conventional practice, it is not only subjected to thrust forces under working conditions resulting from the pressing force and its cross-sectional area, but it is also loaded by flexural stresses.
If these flexural stresses could be obviated, it would be possible, while maintaining the strength of the press stem material, to reduce its cross-sectional area. This can be of particular advantage in indirect extrusion presses. Alternatively, the strength of the press stem material could be reduced while maintaining its cross-sectional area. Furthermore, it would be possible, at a constant level of material strength and cross-sectional area, to increase the length of the plunger, which is also particularly desirable in indirect extrusion presses.
Both with direct and indirect extrusion presses, the requirement arises especially with the "extrusion with shell" and "tube extrusion around a mandrel" processes and also during the entering of the hole, that the press stem should be accurately centered, the tools being in corresponding alignment. Thus, for example, in the case of direct or indirect extrusion presses with formation of a shell, the billet should be subjected to pressure in such a way that the outer zones of the billet will adhere to the wall of the billet container. A shell with uniform wall thickness over its length and periphery can only be formed when the pressure disc (at the end of the stem) is exactly concentric in the bore of the billet container in the case of direct extrusion presses, or that the die complies with this condition in the case of indirect extrusion presses. Only when the pressed shell is of adequate and uniform wall thickness is it possible to avoid metallic oxides and impurities on the surface of the billet from being drawn into the extruded product.
With direct and indirect extrusion presses for tubes around a mandrel, especially with "tube extrusion with shell," it is essential for the attainment of close tube tolerances, that the mandrel should be exactly concentric with the billet container on the one hand and with the die on the other.
With direct tube extrusion presses, the mandrel is guided within the press stem. Consequently, concentrically locating the stem during tube extrusion also ensures accurate guiding of the mandrel.
With indirect tube extrusion processes, the mandrel is guided in a bore of an obturating member centrally mounted in the billet container. The die in this case is mounted in the end of the stem. In this case also, the stem must be able to move exactly concentrically in the bore of the billet container.